Thanks Dean, that's the info I was looking for.

I don't have any shielded cable handy right now. But I did make up a new spindle cable with twisted wires (It's only 12" long), and I grounded the spindle, as it wasn't grounded in the video. But it didn't make any difference.
It just seems to me that I'd see a change in the analog voltage, with a 25-30Hz swing in the frequency?

Since this is just a test, I'll revisit it later when I actually build the control box.

I've got an RS485 converter being delivered today, and will try the UCCNC Huanyang "modbus" plugin. It only cost me $7.